Mechanical changes in living tissue correlate with pathological changes. As between healthy and pathological tissue, the shear elastic modulus (stiffness) and viscosity can vary significantly. With the advent of ultrasound elasticity imaging development over the past decade, many clinical studies have shown that tissue visco-elastic properties provide useful information to physicians for better cancer diagnosis and therapy assessment.
Shear Wave Dispersion Ultrasound Vibrometry (SDUV) is an acoustic radiation force based technique that measures tissue shear elasticity and viscosity by characterizing shear wave speed dispersion, i.e., by frequency. An application of this technique is the non-invasive measurement of liver stiffness to stage liver fibrosis and cirrhosis.
Interrogation by ultrasound, for purposes of medical imaging, often makes use of longitudinal waves. In body tissue, the ultrasound propagates in wave form. In effect, particles all along the propagation path vibrate, in place, back and forth, and the vibration occurs in the direction of propagation. The vibrations create compressions and rarefactions. These are modeled as the peaks and valleys of a sinusoid. Energy is conveyed to the target and back by means of the oscillatory particle movements.
An ultrasound shear (or transverse) wave, by contrast, is characterized by back and forth in-place movement that is perpendicular to the direction of propagation. Oscillation one way creates the peaks, and the other way creates the valleys.
Performing SDUV entails issuing a series of focused longitudinal-wave push pulses. They are high intensity, narrow bandwidth signals which are fired with a repetition rate of (say) 100 Hz. After several of these pulses are fired, each laterally coinciding and in the same direction, they establish a shear wave which propagates out from the focus and in a direction perpendicular to that of the push pulses. The focal depth has been selected so that the shear wave travels through a region of interest (ROI).
A longitudinal-wave tracking pulse is issued to the ROI to assess, at the sampling point, the amplitude of the shear wave. This measurement is used in estimating the phase of the shear wave at the sampled location.
To sample another location, another push pulse issues to the same pushing focus, followed by a tracking pulse to that location. This second cycle is needed, because the difference in phase between two points is used in the determining of elasticity and viscosity.
Shear waves with frequencies of typically 100 Hz and harmonics (200 Hz, 300 Hz, 400 Hz), i.e., components (or “monochromatic shear waves”), are present because the envelope of the pushing pulse is a square wave. The speed estimations at the different frequencies are used in deriving tissue shear elasticity and viscosity.